Merely setting the AutoSize property of the RadWindow to true is not always sufficient for this functionality to work as expected.

The content placed in the RadWindow needs to have explicit dimensions set in pixels. This is necessary, as otherwise you get a confrontation of concepts - a block element (such as the asp Panel, or div, or table) by default takes the width of its parent - 100% of it.
This means that it tries to autosize according to the RadWindow. In the same time we want the RadWindow to autosize itself according to this element, which leads to this obvious contradiction.

This is the reason why some explicit dimensions must exist in the content. Therefore one of the best solutions is adding an empty div with no content or borders that has explicit width - it will not be visible, yet it will allow the RadWindow to autosize correctly.

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